It's actually true. Travel to/from the UK requires a passport check, travel between Schengen countries does not.
Now try to enter LAX or JFK with your national ID and without a visa...
UK/Schengen and Schengen/Schengen are so close compared to EU/Rest of the world, that the differences are imperceptible.
But they are so much perceptible, when you live inside Shengen. Nobody says it is complicated (as an absolute) to go to the UK. But it is more complicated (as a comparison) than travelling within the EU. I don't even understand how that can even be in question.
Just saying that it very strongly depends on the person at border control at the airport and a lot of different circumstances, you can get pretty tight checks from EU to EU too.